This invention relates to fuel cells and more particularly to methods and apparatus for more efficiently and effectively changing the state of operation of a fuel cell, such as shutting a fuel cell down.
Many types of fuel cells are known in the art, such as solid oxide fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells and proton exchange membrane (PEM) fuel cells. Conceptually, the operation of a fuel cell is very simple. An electrolyte separates an anode and a cathode, between which electricity is produced when a fuel is introduced to the anode, an oxidizer is introduced to the cathode, and the cell is maintained within a proper temperature range. The electrolyte allows an ionic species to travel between the cathode and the anode. The reaction products are relatively simple and benign, typically including water and carbon dioxide, thus minimizing environmental concerns, and the operating efficiencies are theoretically relatively high. In contrast to other energy sources, such as internal combustion engines, fuel cells are simpler, less noisy, do not pollute, and create electricity directly. Accordingly, fuel cells are considered by many to hold considerable promise as the power source of the future.
In reality, however, fuel cell systems can be relatively complex, as considerable hardware can be required to support the fuel cells. Such hardware can include thermal management subsystems, fuel processing subsystems, and water management sub-systems. Furthermore, in practice, particular detailed procedures for changing the state of operation of the fuel cell, such as shutting the cell down, or starting the cell up, are often required to avoid damage to the fuel cell.
For example, it can be necessary or desirable to shut down the fuel cell for any number of reasons. An emergency shutdown can be required because a monitored parameter is out of an acceptable range, or a shut down can be scheduled for maintenance of the cell or of the associated subsystems, or servicing of the load may necessitate a xe2x80x9chot hold.xe2x80x9d Each of these state changes typically involves immediately removing the electrical load from the fuel cell.
Such state changes can damage the fuel cell if electrode potentials, which typically rise upon removal of the load, are not properly controlled when the cell is at an elevated temperature. For example, as disclosed in U.S. Pat. No. 5, 045, 414, issued to Bushnell et al. and herein incorporated by reference, in a phosphoric acid fuel cell the cathode can undergo catalyst dissolution, catalyst support corrosion and catalyst layer flooding, if the potential exceeds eight tenths (0.8) of a volt. On the other hand, if the cathode potential approaches the anode potential and the cathode is subsequently re-oxidized, the catalyst can recrystallize and lose activity. If the anode reaches the potential of the cathode, it can flood with electrolyte. Accordingly, the control of electrode potentials is an important concern.
The ""414 patent discloses a variation of a technique, known in the art, of purging and passivating the fuel cell upon removal of the service load. According to the ""414 patent, nitrogen is added to the cathode flow field to create a nitrogen/oxygen mix having less than 1% oxygen by volume, and nitrogen is supplied to the anode. The concentration of oxygen supplied to the cathode is important, as low a concentration can prevent achieving the proper potential and too high a concentration can cause catalyst dissolution, corrosion of the catalyst support, and catalyst layer flooding. The proper concentration of oxygen can depend on several factors, including the matrix thickness and composition of the electrolyte, as well as the temperature of the fuel cell. A dummy load is often switched to the cell to bring the cathode potential down rapidly at the start of the purges.
However, such approaches are not entirely satisfactory. The possibility of damage to electrodes remains due to the relatively sudden and large change in electrical loading on the cell. Furthermore, reconnecting the service load after a xe2x80x9chot holdxe2x80x9d requires purging the system of the inert gases and reintroducing the reactants at the appropriate rates and concentrations to achieve damage-free normal power operation.
As a simpler and more effective technique for changing the state of operation of a fuel cell would be a welcome advance, it is an object of the invention to improve upon the aforementioned methods and apparatus of the prior art, and to address one or more of the disadvantages and drawbacks thereof.
It is another object of the invention to provide improved methods and apparatus for changing the state of operation of a fuel cell.
Other objects of the invention will in part be apparent and in part appear hereinafter.
It is understood that it is undesirable for a fuel cell to be at an elevated temperature and under little or no electrical load. Yet in practice, changing the state of operation of a fuel cell, such as shutting the fuel cell down, typically involves immediately applying a small load, such as a dummy or open load to the fuel cell (i.e., disconnecting the fuel cell from the service, or normal, electrical load) and initiating a cool down and purging procedure.
According to the invention, it is understood that in particular between a temperature range of approximately the operating temperature of the fuel cell and a transition temperature lower than the operating temperature, the application of an idle electrical load to the fuel cell can advantageously provide a simpler technique for changing the state of operation of the fuel cell and for preventing damage to the fuel cell. During application of the idle load, fuel and oxidizer input reactants are provided commensurate with the power delivered to the idle load. Below approximately the transition temperature a lesser load, such as dummy load or an open load, may be applied without substantial harm, and typical passivating/purging procedures followed that can include more severely limiting the flow of one or both of the input reactants and introducing gases such as nitrogen of mixtures of nitrogen and other gases.
In one aspect, the invention provides a method of operating a fuel cell system having at least one fuel cell to transition the fuel cell between a normal state of operation at a normal operating temperature and under a service load and a second state at a second temperature less than the operating temperature and under a to second load less than the service load. The method includes the steps of:
(a) initiating one of a warm-up and a cool down procedure for changing the temperature of the fuel cell;
(b) applying to the fuel cell an idle load that is less than the service load and greater than the second load, the idle load being applied when the temperature of the fuel cell is between approximately the normal operating temperature and a transition temperature;
(c) applying to the fuel cell one of the service load and the second load, the second load being applied when the temperature of the fuel cell is less than approximately the transition temperature and the service load being applied to the fuel cell when the temperature of the fuel cell is one of equal to or greater than approximately the normal operating temperature.
In a further aspect, the invention provides a method of operating a fuel cell system having at least one fuel cell to transition the fuel cell between a normal state of operation at a normal operating temperature and under a service load and a second state at a second temperature less than the operating temperature and under a second load less than the service load. The method includes the steps of:
(a) removing the service load from electrical connection to the fuel cell;
(b) applying to the fuel cell an idle load that is less than the service load and greater than the second load;
(c) reducing the temperature of the fuel cell to a transition temperature;
(d) removing the idle load from electrical connection to the fuel cell
(e) applying the second load to the fuel cell,
(f) initiating a purging/passivation procedure on the fuel cell; and
(g) reducing the temperature of the fuel cell to the second temperature.
In yet another aspect, the invention provides a fuel cell system that includes at least one fuel cell; a cooling system for circulating a cooling fluid for controlling the temperature of the fuel cell; a temperature sensor for monitoring the temperature of one of the cooling system coolant and the fuel cell for communication of an electrical signal responsive to the temperature over a databus; a load control means for selectively applying an idle load to the fuel cell and applying a service load to the fuel cell; and a processor for controlling the transition of the fuel cell between a normal state of operation at a normal operating temperature and under a service load and a second state at a second temperature less than normal operating temperature.
The processor can include an input/output element for communication over a databus; a memory element; and a program element for execution by the processor. The program element can include a first means for communication with the temperature sensor over the databus for receiving the electrical signal for monitoring the temperature of the fuel cell; a second means for controlling the cooling system to transition the temperature of the fuel cell between the normal operating temperature and a second temperature less than the normal operating temperature; and a third means responsive to the first and second means for controlling the load control element for applying the idle load to the fuel cell when the temperature of the fuel cell is in transition and between approximately a normal operating temperature and a transition temperature and for applying the service load to the fuel cell when the temperature of the fuel cell is at approximately the normal operating temperature.
As is understood from the use herein, the term xe2x80x9cloadxe2x80x9d refers to the amount of electrical power drawn from the fuel cell 18. A large load draws more power that an small load, though a small load presents a higher impedance than a large load. For consistency, an xe2x80x9copenxe2x80x9d circuit, which has infinite impedance and hence draws no power, is also referred to as a load. Accordingly, the term load does not refer to any one physical entity for consuming power; applying a certain load to the fuel cell means that a certain amount of power is drawn from the cell, regardless of what physical entities are electrically connected to the fuel cell for receiving that power. As used herein, a normal, or service, load is that load that is typically applied to the fuel cell when the cell is at its normal operating temperature. As understood by one of ordinary skill in the art, a fuel cell system typically includes a plurality of fuel cells assembled together such that the electrical outputs of the fuel cells are combined. Accordingly, the absolute magnitude of a xe2x80x9cnormal,xe2x80x9d xe2x80x9cidlexe2x80x9d and xe2x80x9cdummyxe2x80x9d loads will typically vary according to the number of fuel cells electrically combined. For simplicity, however, reference is made herein to a single fuel cell as representative, with the understanding that the typical fuel cell system includes many fuel cells, and that the disclosure herein is to apply to such systems. The normal operating temperature of the fuel cell can vary with the type of fuel cell.